The present invention relates to a method for determining the number of defective digital bits (defective bit number) of the sequence that has been transmitted over a data-transmission path to be tested. Specifically, in the method of the present invention, a test sequence consisting of digital data (bits) is provided to an input end of the data-transmission path, an output sequence received on an output end of the data-transmission path is cross-correlated with a reference signal corresponding to the test signal or the inverted test sequence, and the number of defective bits is determined by evaluating the extreme (i.e., the maximum or minimum value having the greatest absolute value) value of the cross-correlation function thus formed.
In the German patent publication no. A1 40 12 850 ("the '850 patent"), a test sequence formed from a sequence of digital data is fed on the input end of a transmission path. Digital data having a self-correlation (i.e., auto-correlation) function which comprises a pronounced extreme value, for instance in the form of a Dirac pulse (for the definition of Dirac pulse, see Otto Mildenberger "Grundlagen der Systemtheorie fur Nachrichtentechniker" {Principles of System Theory for Communication Engineers}, Hansa-Verlag, 1981, pages 48-50) are preferably used as the test sequence. In the method of the '850 patent, an output sequence is received at the output end of the transmission path based on the test sequence transmitted. To form a cross-correlation function, the output sequence is cyclically cross-correlated, bit-wise over its length, with a reference sequence which corresponds to the test sequence or the inverted test sequence. The term "inverted test sequence" refers to a sequence which consists, in each case, of the complementary digital data of the test sequence.
The magnitude of the extreme value (i.e., the maximum or minimum) of the cross-correlation function is evaluated to determine the defectively transmitted data (number of defective bits). Fundamentally, an inverse relationship exists between the absolute amount of the extreme value and the number of defective bits within a relatively wide range.
In the method of the '850 patent, to indicate the number of defective bits, the extreme value must be subjected to further arithmetic operations, in the form of a subtraction and a division. If, for instance, a binary sequence of numbers in the form of a 2.sup.5 -m sequence EQU C.sub.o (n)=0000100101100111110001101110101 (Equation 1)
with a period of N=31 is used as a test sequence and a reference sequence after converting the sequence, by translating the binary values 0 and 1 to the bipolar values -1 and +1, into a bipolar numerical sequence of the form ##EQU1## Further, if an output sequence h.sub.bi is falsified, for instance, in one place (bit position), the cross-correlation function (KKF(n).sup.-) is formed in accordance with the equation ##EQU2## Thus, with the method of the '850 patent, an extreme value KKF.sub.Ext of 29 is obtained. To indicate the number of defective bits BF based on the extreme value KKF.sub.Ext, the following arithmetic operations are necessary EQU BF=(N-KKF.sub.Ext)/2=(31-29)/2=1 (Equation 4)
These arithmetic operations, which are necessary for indicating the number of defective bits, require a considerable circuit expenditure since they cannot be realized by simple gate circuits. The required arithmetic operations reduce of the speed of the operation of the method of the '850 patent.
The object of the present invention is to provide a method for determining the number of defective digital data transmitted over a data-transmission path to be tested having a high process speed and a simple way of indicating the number of defective bits ascertained.